A predominant number of color picture tubes in use today have line screens, and shadow masks that include slit-shaped apertures. The apertures are aligned in columns, and the adjacent apertures in each column are separated from each other by webs or tie-bars in the mask. Such tie-bars are essential in the mask, to maintain its integrity when it is formed into a dome-shaped contour which somewhat parallels the contour of the interior of a viewing faceplate of a tube. Tie-bars in one column are offset in the longitudinal direction of the column (vertical direction) from the tie-bars in the immediately adjacent columns. When electron beams strike the shadow mask, the tie-bars block portions of the beams, thus causing shadows on the screen immediately behind the tie-bars.
When the electron beams are repeatedly scanned in a direction perpendicular to the aperture columns (horizontal direction), they create a series of bright and dark horizontal lines on the screen. These bright and dark horizontal lines interact with the shadows formed by the tie-bars, creating lighter and darker areas which produce a wavy pattern on the screen, called a moire pattern. Such a pattern greatly impairs the visible quality of the image displayed on the screen. Analysis of moire in shadow mask tubes, using Fourier analysis and geometrical considerations, shows that the visibility of moire depends mainly on the amplitude and pitch of the moire pattern. Moire amplitude depends on the vertical spot size and tie-bar width. Moire pitch depends on the interference between the periodic repetitivity of tie-bar alignment and the period of scanning lines.
It is highly desirable to select a shadow mask tie-bar spacing and sizing that will minimize the moire pattern for any scan condition used in the television receiver. The industry change from one-mode to multistandard television receivers complicates the selection such that it is necessary to reach some compromise to achieve acceptable moire for all multistandard modes. The two scan conditions presently in use are interlaced scan and non-interlaced scan. The following Table presents the standards (interlaced and non-interlaced) that were considered in developing the present invention.
TABLE ______________________________________ VISIBLE VISIBLE LINES LINES STANDARD OVERSCAN PAL/SECAM NTSC ______________________________________ 4/3 107% 537 (268) 453 (227) &lt;4/3&gt; 119% 483 (241) &lt;&lt;4/3&gt;&gt; 137% 420 (210) 359 (180) 16/9 75% 716 (358) &lt;/9&gt; 83% 647 (324) ______________________________________
In the Table, 4/3 and 16/9 represent the horizontal-to-vertical aspect ratios of the screens. The second column, labeled Overscan, presents the amount of vertical overscan of the 4/3 standard transmissions and the amount of vertical underscan of the 16/9 standard transmissions. In the 16/9 standard, there is a corresponding amount of overscan in the horizontal direction to obtain the 16/9 ratio. The third column presents number of visible lines in the standard PAL/SECAM transmission and the fourth column presents the number of visible lines in the standard NTSC transmission. For example, in the PAL transmission with 625 lines and 107% overscan, there will be 537 visible lines on the screen. The standards &lt;4/3&gt; and &lt;&lt;4/3&gt;&gt; are related to two zoomed modes, respectively, of 119% and 137% enlargement. Because of the enlargement, the number of viewed lines on the screen are less. Similarly, &lt;16/9&gt; is an enlarged mode of the 16/9 standard. The numbers in parenthesis indicate the non-interlaced modes. In actual television receivers, the modes to be considered for teletext transmission are only 268 lines for PAL and 227 lines for NTSC, but for moire calculations it is useful also to consider the non-interlaced modes to account for the possibility of improper interlace in a receiver, which would produce some moire.
There have been many techniques suggested to reduce the moire problem. Most of these techniques involve either adjusting the vertical size of the electron beam spot at the screen, such as by modifying the electron gun and yoke, or rearranging the locations of the tie-bars in the mask to reduce the possibility of the electron beam scan lines beating or interacting with the tie-bar shadows. U.S. Pat. No. 4,751,425, issued to Barten on Jun. 14, 1988, shows a mask wherein the tie-bars are located on straight lines that form an angle of between 3 and 8 degrees with the horizontal direction of deflection. Although techniques, such as that shown in the Barten patent, have been used successfully in the past to achieve some reduction in moire, they concentrate on correcting primary moire pitch and do not consider secondary moire pitch that arises when the inclination of tie-bar lines is introduced. Therefore, there is still a need for improved moire reduction techniques which consider the secondary moire pitch. Such improved techniques are needed especially for the newer higher quality color picture tubes that are required for higher definition television. For example, as the quality of electron guns improves to meet the needs of higher definition television, such improved guns produce smaller electron beam spots at the screen. This reduction in electron beam spot size produces visually sharper scan lines on the screen which interact with the tie-bar shadows and increase the moire pattern visibility problem.